Interpreting the Test Results

For the Ford AC Generator with Built-in Voltage Regulator

This web site is mainly devoted to helping you diagnose and repair electrical problems by understanding how the circuits work, but if you're still confused or are having difficulty finding the defect, here's a more direct approach with measurements you need to take, and how to analyze them.

Most charging system problems can be diagnosed with an inexpensive digital voltmeter available from auto parts stores, department stores, and home supply and hardware stores. Professionals use expensive load testers to obtain more information but even those only provide numbers that must be interpreted to diagnose problems.

We know from "Test Your Charging System" that system voltage must be maintained between 13.75 and 14.75 volts. Your results will show low voltage, high voltage, or voltage that drops at idle. Analyzing professional test results will also be explained.

Low System Voltage (No Output From Generator)

A fully charged battery will measure near 12.6 volts with the engine off and no loads turned on. Once the engine is started, the voltage must immediately rise to at least 13.75 volts. When no change is observed, it will be necessary to split the system into the low-current input circuit and the high-current output circuit. Most problems will occur in the input circuit. Two voltage measurements will start the troubleshooting procedure, then, if appropriate, a full-field test will add to the diagnosis.

Every AC generator has two carbon brushes that transfer field current to the rotating field winding, also called the "rotor". With high mileage it's common for those brushes to wear down leading to an open circuit. That almost always starts out as an intermittent problem. You'll see the volts gauge on the instrument cluster drop at times and return to normal at other times. Normally problems in the output circuit are permanent. They don't resolve themselves and work once in a while. Ford has one of the exceptions due to the design of their stator winding.

Figure 1 shows the two different types of stator windings. Most AC generators use the delta winding shown on the left. You can identify this type by the two wires at each connection to its pair of diodes. If a wire were to break off one of the connections, two thirds of the generator's capacity would be lost. Ford uses the "wye" winding shown on the right. A brass rivet is used to hold the three windings together at their junction. This stator has only one wire at each connection to a pair of diodes. With two different types of metal, (brass and copper), and an acid, (road salt), "galvanic action" takes place leading to corrosion. Galvanic action is what takes place in the car's battery with two different forms of lead and an acid. When this connection corrodes apart, it is possible to have no output. The input circuit can be verified as working with voltage measurements.

Figure 1. Every AC generator uses one of these types of stator windings.

Figure 2. Charging system circuit.

Figure 2 shows the basic circuit with the voltage regulator bolted to the generator's rear housing. The "A" and "F" test points are clearly marked.

Figure 3. Normal system voltages with the ignition switch off.

Figure 3 shows the voltage regulator on the back of the generator and the normal voltages at all test points with the ignition switch turned off.

Output Circuit

The output wire, shown in red, always has 12 volts present on it. It is typically about an 8 gauge wire so it will be much larger than most other wires. It is the only one bolted to the back of the generator. Expect to find a protective rubber cover over the nut to prevent accidental shorting to ground when you're working in the area.

Full battery voltage must be present at the output terminal at all times as shown by the bright test light in Figure 4. If this voltage is missing, look for a burned fuse link wire or corroded splice on one end of it. Accidentally grounding the generator's output terminal can cause the fuse link to burn open but that takes some time. If the fuse link is burned open when that terminal wasn't grounded, suspect shorted diodes inside the generator. There are two sets of three diodes, and often an additional pair, and any one diode in each set must be shorted to create a dead short to ground. That is very rare.

Fuse link wires are the "weak link in the chain" and will always be smaller in diameter than the rest of the circuit they protect. Do not replace them with regular wire of the same diameter. The insulation on fuse links is specially designed to not burn or melt when the wire burns. Replacement fuse link wire is purchased by gauge and color to denote its current rating. One piece can be cut to length to perform three or four repairs.

It is not often you'll find the voltage missing at the output terminal. When you do, the most common cause is a wire that got overlooked when the starter relay was replaced. That relay is mounted on the inner fender near the battery. The large "battery" terminal on the relay is used as a convenient tie point for other circuits coming right from the battery.

On later models that output wire is bolted to the under-hood fuse box. Loose connections there are fairly common but they might not show up with a test light as shown in Figure 4. The test light draws very little current. That current can easily make it through that poor connection, but the higher generator output current can't. That's when the problem occurs. That loose connection will show up with voltage tests when the engine is running. We'll check for that later, if necessary.

That's it for the output circuit. One quick test with the test light. In this case it will be more accurate than an expensive digital voltmeter. A voltmeter could give misleading results if the fuse link is burned open. For an explanation, see "Why Doesn't This Circuit Work?".

Input Circuit

Leave the ignition switch off for these tests. More problems occur in the input circuit because there are more connections and parts that can fail. Start by testing for voltage on the "A" terminal on the back housing of the generator. A bright test light means that circuit is okay. If voltage is missing, back-probe the yellow wire through the rubber seal. If voltage is present there, the connector terminal is not making a good connection. If voltage is missing, check at both sides of the two-wire connector. If it's still missing there, the fuse link is burned open.

Figure 5. 12 volts at the "F" terminal proves the brushes are okay.

If you have 12 volts at test point "A", check at test point "F". 12 volts at test point "F" proves the circuit is okay up to the voltage regulator. If you find 0 volts there, use an ohm meter to verify a break in the circuit between the two field terminals. Wiggle the pulley if necessary since it's common for dirt and carbon particles to cause a poor connection when the rotor isn't spinning, but an open circuit means one of the brushes is worn. They are both replaced as part of an assembly that is bolted to the voltage regulator. On some engines the regulator can be replaced without even removing the generator from the engine.

Battery Light Circuit

The voltage regulator is the only thing stopping current from flowing through the field winding when the ignition switch is turned off. When the switch is turned on, a small current flows through the warning light on the dash to the voltage regulator on the green / red wire. That is the "wake-up" signal that turns the regulator on. That lets current flow through the field circuit. There is very little resistance to ground from the "I" terminal so the current is sufficient to cause the dash light to turn on.

Figure 6. Ignition switch is turned on.

Figure 6 shows the ignition switch on and the dash light lit up. You'll find about 2.0 volts at the "I" terminal. Back-probe it through the rubber seal the wire goes through.

If the dash light does not turn on, measure the voltage on the green / red wire at the regulator's plug. If there's 0 volts, there's a break in that circuit. Stretched or corroded terminals in the two-pin connector is the most likely suspect. Full battery voltage indicates an open connection right there in the regulator's plug, or the regulator is open and must be replaced. Low voltage on the green / red wire means the circuit is working and the generator should start working when the engine is started. The dash bulb is burned out, but there is a resistor across its socket that will pass enough current to wake up the regulator.

The last possibility is there is 0 volts at the "I" terminal and the dash light is on. The green / red wire would have to be grounded. No current will go through the regulator to wake it up. Performing the full-field test will verify the rest of the generator is okay. The system should work properly when the short on the green / red wire is repaired.

Figure 7. Field current path shown in purple.

Figure 7 shows the field circuit with the current path outlined in purple. When the wake-up signal is received at terminal "I", current leaves the battery's positive post, travels through the first fuse link, a splice, (good place to find a corroded connection), then the yellow wire continues through another fuse link, through the two-pin connector, to the regulator's plug.

The "A" terminal is connected internally directly to the first brush at the "A" screw. This circuit also provides the current to run the voltage regulator, and it is where the regulator senses system voltage.

From the "A" screw, current flows through the field winding, out the second brush to the "F" screw, then finally to the voltage regulator's circuitry. Next, the closed switch represents the regulator circuitry that is turned on to complete the circuit to ground. That circuitry changes resistance to control how much current flows through the field winding thereby controlling system voltage.

Figure 8. Voltage induced in the stator.

As soon as the engine is started, the rotating magnetic field induces voltage in the stator winding. Figure 8 shows the stator and one of the six or eight diodes. Output current goes back to the battery and to other electrical systems. The voltage on the "F" terminal is considerably lower than that on the "A" terminal indicating current is flowing through the field winding and it is dropping some voltage.

Figure 9. Voltage on the white / black wire tells the regulator output is being developed.

Now that an output is being generated, a sample of that voltage is tapped off one of the stator coils, (the top right one in Figure 9), and goes to the "S" terminal in the voltage regulator's plug. That is the white / black wire. The exact voltage is irrelevant but it is typically around half of system voltage. When it appears, the regulator knows the system is working and it puts 12 volts back onto the green / red wire going to the dash light. With 12 volts on both sides, the warning light turns off. Ford has used this stator wire since the mid 1960s.

Keep in mind we're using 12 volts here to represent system voltage to simplify circuit explanations. You should find 13.75 to 14.75 volts on the car you're working on.

Figure 10. Full-field procedure.

Full-Field Test

A fast way to eliminate everything but the voltage regulator and prove the rest of the system is okay is to perform the full-field test. Ford made that very easy by identifying the "F" screw. With the engine running, use a jumper wire to ground that screw as shown in Figure 10.

The dash light is on indicating a problem, and system voltage was found to be less then 12.6 volts. If system voltage goes up during the full-field procedure, that proves the brushes and the rest of the input circuit are okay. Either the regulator is open or it isn't getting the wake-up signal on the green / red wire. You'll have to go back to the previous "Input Circuit" section and perform those tests.

WARNING: Do not raise engine speed if the full-field test proves the generator is working. With the voltage regulator bypassed in this test, it is possible for system voltage to reach excessive levels. With increased engine speed, 30 volts or more is possible. That will burn out any light bulbs that are turned on, and can destroy the radio, computers, and diodes inside the generator. The full-field test should only be performed long enough to verify generator operation.

If voltage doesn't rise during the full-field test, check for continuity between the "A" and "F" terminals and for voltage on the yellow "A" wire.

Output Circuit

Without performing a load test it's hard to know if there's a bad diode in the output circuit. Unlike other systems, this voltage regulator will turn on the dash light in response to an undercharge and an overcharge condition. Overcharging is the result of a shorted voltage regulator. Loss of the system voltage sensing circuit is not suspect because that is the same circuit that powers the regulator. If there's a break in that circuit, the regulator won't turn on so it can't overcharge.

Undercharge is a sign of a defective diode. The generator's maximum output will be one third of its normal capacity. Its output voltage will drop when it can't meet the current demands of the vehicle, particularly at idle.

One final test involves loose or dirty connections at the under-hood fuse box on newer cars or starter relay on older models. Any resistance in those connections will severely limit the amount of current that can get through. That will result in low system voltage and unusually high voltage at the generator's output terminal. Compare voltages at the battery positive post and the generator's output terminal. If battery voltage is lower, there's excessive resistance in the wire between those two points.

Professional Tests

Most shops will provide a printout or description of their findings that will include "full-load output current", "voltage regulator setting", and "ripple". Generators do not get weak so your output current will either be normal, no output due to a defect, or one third of normal due to a defective diode. The most common generators of this time period were capable of up to 85 – 100 amps. Up to three of those amps goes right back to run the field winding and they may not show up in the test. Not all generators produce exactly the same output so 80 to 110 amps is perfectly acceptable. Generators capable of producing well over 100 amps were also available.

It's been mentioned on numerous occasions that system voltage must be between 13.75 and 14.75 volts. If it is high, suspect the voltage regulator. It is common for system voltage to be too low but rise slowly over a period of many minutes if the battery was previously discharged. The voltage will come up as the battery charges.

If no mention of ripple is made by your mechanic and output current is satisfactory, you can assume it is not excessive. If ripple is excessive and maximum output current is around 25 – 35 amps, suspect a defective diode. Replacement of them can be rather involved and due to the cost of replacement parts, it generally makes more economic sense to replace the entire generator.